The present invention relates to a method for arranging a coating, in particular a hardfacing, to a component, in particular a TiAl engine part, and a corresponding engine component provided with the coating, in particular a rotor blade of a low-pressure turbine.
Turbine blades for low-pressure turbines can comprise shrouds, which mutually abut each other adjacent to one another. The adjacent lateral surfaces are normally configured to be Z-shaped and have contact regions, in which the shrouds directly border each other in order to contribute to vibration dampening. These contact surfaces of the shrouds are normally provided with a hardfacing to keep the mechanical abrasion low. According to the prior art, Co—Cr alloys are used for this, in particular so-called Stellites (registered trademark of the Deloro Stellite company), which are applied for example by WIG welding, microplasma welding or laser beam welding or by other application welding processes. Whereas this type of hardfacing is well suited for nickel-based alloys or superalloys, it is problematic in the case of turbine blades made of titanium aluminides (TiAl alloys), because the intermixing of TiAl with Stellites causes brittle phases to develop that can lead to the formation cracks.
For this reason, plasma-sprayed layers of the Co—Cr alloy T-800 (registered trademark of the Deloro Stellite company) were used with TiAl blades for low-pressure turbines. However, in some circumstances these coatings or hardfacings do not satisfy the requirements for adhesive properties. Accordingly, applying molded parts made of Stellites using soldering continued to be proposed (WO 2011/009430) for hardfacing the contact surfaces of shrouds of TiAl low-pressure turbine blades (the so-called Z-notches).
However, disadvantages arise here to the effect that the molded parts need to satisfy very high requirements for dimensional accuracy in order to ensure a full-surface and precise contact of the molded part on the component to be coated. This makes corresponding molded parts made of Stellites relatively expensive.
Therefore, the object of the invention is to avoid the disadvantages of the prior art and make a hardfacing on a TiAl engine component possible, in particular a TiAl low-pressure turbine blade, wherein the coating should be simple to carry out and should supply reliable results with respect to a well adhering hardfacing.
Advantageous embodiments are the subject matter of the dependent claims.
The invention proposes a new coating method for producing a hardfacing on a Z-notch of low-pressure turbine blades made of TiAl, in which a green compact is arranged on the component to be coated with the to-be-coated material (coating material) with the presence of a solder and the coating is formed in the form of a sintered body by means of a combined solder-sintering process and the coating is connected to the component. Combining the soldering and sintering into one process step produces a simple production possibility with a low expense, while simultaneously ensuring that a metallurgic bond and full-surface contact of the coating of a component being coated are guaranteed.
This is ensured by providing a solder, whereby the solder can already be contained in the green compact, i.e., in the to-be-sintered mold body of the coating material. In particular, the solder can be present in a graded manner in the green compact so that for example, on the side, in which the green compact is arranged on the component to be coated, the percentage of the solder is high and declines with increasing distance from the component to be coated.
Alternatively or additionally, the solder can also be provided by a slurry, in which the solder is incorporated by means of a binding agent and/or solvent. Because of the binding agent and/or solvent, the soldering material is able to be arranged on the component to be coated in a simple manner by applying the flowable slurry, for example by means of coating, spraying or the like.
The slurry can also comprise an adhesive in order to ensure a good adhesion of the green compact and/or of the slurry on the component to be coated when arranging the green compact on the component to be coated by means of the slurry.
The solder is contained in the slurry in the form of a powder or in the form of particles, wherein the particles can be selected to be very fine-grained in order to ensure both thin slurry coats as well as full-surface contact of the coating on the component to be coated. Correspondingly, the particle size of the solder in the slurry can be less than or equal to 50 μm, preferably less than or equal to 25 μm. In this case, the particle size can be selected in the form of an average particle size or in the form of a maximum particle size.
The binding agent and/or solvent can be an organic binding agent and/or solvent, e.g., a screen printing oil, which guarantees a uniform and well adhering distribution of the slurry and therefore of the solder on the component to be coated.
The green compact, which comprises the coating material in the form of particles and/or the solder, likewise in the form of particles, can have a thickness of 0.2 mm to 2 mm, preferably 0.3 mm to 0.6 mm in order to form the coating or hardfacing by sintering the particles from the coating material.
After applying the slurry to the component to be coated and/or applying an adhesive to the green compact and arranging the green compact on the slurry layer, the binding agent and/or solvent and/or an adhesive can be dried in a first thermal treatment, wherein the component to be coated is heated with the slurry and the green compact locally or as a whole to temperatures in a range of 60° C. to 100° C. This results in a preliminary fixing of the green compact on the component to be coated.
Thereafter, the combined solder-sintering process can be carried out with correspondingly high temperatures, in which the binding agent and/or solvent vaporize, the coating material in the green compact sinters and the solder in the former slurry layer and/or in the green compact melts. The thermal treatment can be realized in particular by inductive, local heating of the coating region. After the combined solder-sintering process, the particles of the coating material in the green compact are formed into a sintered body, which forms the coating or hardfacing and the solder provides a well adhering connection between the sintered body, the particles and the component to be coated.
The solder-sintering process can be carried out in a vacuum, in particular a high vacuum, or under protective gas, for example in an argon atmosphere.
Possible as coating material are Co—Cr alloys, in particular Co-based alloys with a chromium percentage of over 25% by weight and W percentages of 4% to 20% by weight or Co—Cr alloys with Co-based alloys with a Cr percentage of less than 20% by weight and a Mo percentage of greater than 20% by weight. Examples of this are, in particular, the alloy T-800 or the Stellite alloys from the Deloro Stellite company.
The solder for the slurry and/or the green compact can be a nickel-based solder, in particular a SAE Standard AMS4777 solder.